Processes for hydrogenating carbon-carbon double bonds of a conjugated diene polymer to produce a highly saturated polymer are known. Various homogeneous catalysts and heterogeneous catalysts have been proposed for use in these hydrogenation processes (for example, see Japanese Unexamined Patent Publication [hereinafter abbreviated to “JP-A”] No. S58-17103, JP-A S59-117501, JP-A S61-247706 and JP-A S62-42937).
Of these hydrogenation catalysts, those which contain a platinum group metal represented by palladium or rhodium are popularly used because they exhibit a high activity for hydrogenation of carbon-carbon double bonds of a polymer chain and the hydrogenation can be achieved with a high hydrogenation percentage and a high selectivity. For example, a catalyst comprised of palladium or rhodium supported on a carrier is used in an industrial scale for hydrogenation of an acrylonitrile-butadiene copolymer, which is one kind of the conjugated diene polymer.
However, production of platinum group metals such as palladium and rhodium is limited, and these metals are scarce and expensive. Therefore, it is eagerly desired to reduce the amount of the platinum group metals used, namely, to more enhance the catalytic activity from a viewpoint of cost reduction and conservation of natural resources. More specifically it is requested for enhancing the efficiency in the above-mentioned hydrogenation of an acrylonitrile-butadiene copolymer in an industrial scale.
Various methods have been proposed for the preparation of the platinum group metal catalysts. For example, in the case of a supported palladium catalyst, a process is generally adopted wherein a palladium compound capable of being converted to metallic palladium is dissolved in an appropriate solvent such as water or acetone; the thus-obtained solution is sprayed onto an appropriately selected carrier or the carrier is dipped in the solution, and then the carrier is dried; the palladium compound supported on the dried carrier is converted to oxide or hydroxide by a treatment with an aqueous solution of sodium hydroxide or sodium metasilicate, and finally the oxide or hydroxide is treated with a reducing agent such as hydrazine or hydrogen to be thereby reduced into metallic palladium. Alternatively, the palladium compound supported on the dried carrier is treated with the reducing agent without the pre-treatment for conversion to oxide or hydroxide.
As mentioned above, most conventional processes for the preparation of a supported platinum group metal catalyst comprise many complicated steps such as dissolution of a raw material for the catalyst, application of the solution on a carrier by spraying or dipping, drying or burning of the solution-applied carrier, and reduction of oxide or hydroxide with a reducing agent. A problem arises in that activity and service life of the prepared catalyst vary depending upon the particular conditions adopted for the preparation of catalyst.
To enhance the efficiency of hydrogenation, a method using a homogeneous palladium catalyst and a method reducing a palladium compound in a hydrogenation reaction system have been examined. For example, a hydrogenation process using a palladium salt of a carboxylic acid as a catalyst has been proposed in JP-A S59-117501 wherein the catalyst is reduced with hydrogen in a solution of a conjugated diene polymer and subsequently hydrogenation is carried out. A catalyst system for hydrogenation of a conjugated diene polymer, a process for preparing the catalyst system, and a process for hydrogenating the conjugated diene polymer have been proposed in JP-A H6-298844, which catalyst system comprises a polymer complex comprised of a hydrogenation catalyst such as a palladium compound and a specific polymer such as, for example, a nitrile group-containing polymer. Homogeneous palladium catalysts proposed in the patent publications have a problem in that a step of recovering the catalyst after completion of the hydrogenation is complicated.
A process for hydrogenating an acrylonitrile-butadiene copolymer has been proposed in JP-A H2-196803 wherein the copolymer is hydrogenated in a solvent in the presence of a catalyst system prepared by reacting a hydrogenation catalyst such as a palladium catalyst with one or more specific metal compounds such as, for example, an aluminum compound. However, this composite metal type catalyst system does not exhibit sufficiently high activity for hydrogenation.